Pfpd Electronics Circuit Description - Scion Instruments 436-GC Service Manual

436-GC/456-GC

PFPD Electronics Circuit Description

Power supplies and grounds
There are four grounds serving the PFPD electronics circuitry. Ground 1 is the return for the +5V digital
supply. It is connected directly to Ground 4, the unregulated +24V return, at the card edge connector on
the Mother board. The analog return is Ground 2, which provides a low-noise return for the ±15V and
+5.25V supplies. Ground 3 is the reference for analog signal distribution, which carries almost no DC
current. All of the grounds must be tied together externally for the board to function properly.
Three RC filters remove noise from the analog power supplies. These consist of R464 with C47, R47 with
C48, and R48 with C49.
Digital circuits
U11 decodes bus address information to access buffered shift register U10, latch U12 and buffer U14, as
well as a dual DAC. The latch holds all of the digital control signals for the board, while the buffer transmits
the board identification number and status data to the bus.
Commands and data for the on-board microcontroller (U9, an 87C751) are transferred from the bus
through shift register U10. When a byte is directly latched into the register, flip-flop U7 is simultaneously
set. Its output (pin 5) tells the microcontroller that a byte is available. The microcontroller then pulses U10-
13 low to transfer the latched data into the shift register, followed by 8 pulses to U10-11 to shift the data out
through U10-9. When all of the data bits have been read, the microcontroller pulses U7-1 low, clearing the
latch to inform the CPU that the buffer is available for another byte.
Most of the outputs of the 87C751 control analog functions, which may be disturbed by noise on the digital
control lines. VR2 regulates the +5V power for U9, keeping the noise from the system +5V supply off of the
microcontroller's outputs.
The reset signal for the microcontroller comes from latch U12. Software must reset U9 with a positive pulse
lasting at least one microsecond before it will execute its program.
Electrometer
AR2 (pins 1-3) converts the small current from the photomultiplier tube into a voltage. On the most
sensitive range ("Range 10"), both switch sections 1 and 2 (pins 2-3 and 6-7) of U3 are open, leaving R21
as the only feedback resistor. The effective feedback resistance is reduced on ranges 9 and 8 by closing
switch section 1 or 2, respectively, placing either R19 or R20 in parallel with R21, for feedback resistances
of 2M or 198k. These resistances produce nominal full-scale output voltage (10mV at AR1-1) at currents of
-10 -9
5x10 , 5x10 , and 5x10
Range settings.
The range setting is directly controlled by the microcontroller (U9), remaining static only when the user
selects Range 10. On the less sensitive ranges, AR2 is set to the most sensitive range while waiting to
detect the start of a flame pulse, changing to the selected range setting only during the integration time.
Since the microcontroller samples the user range selection only once during each flame pulse cycle, the
setting at AR2 will not reflect changes in the method until the next pulse occurs.
Capacitor C19 sets the time constant of the electrometer to 100µs on Range 10. This is about 1000 times
faster than the standard FPD electrometer, in order to follow the rapid changes in current during the flame
pulses. Since the noise is low on the less sensitive ranges, no extra capacitors are used to set the time
constant on these ranges, and the time constant falls to 10µs and 1µs on Ranges 9 and 8, respectively.
A switchable current source is provided at the electrometer input to test entire signal path. As long as latch
output U12-12 remains low, switch U8 (pins 6-8) is turned on. This holds Q1 off, disconnecting the
diagnostic current from the electrometer input. Turning off U8 allows the emitter of Q1 to drop to -600mV,
placing 9.4V across R29 and injecting 470nA into the input.
SCION Instruments
-8
on the three ranges, which are 5 times higher than the currents implied by the
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 95